1. Field of the Invention
The present invention relates to a spacial light modulation device and a method of manufacturing the same, and more specifically to a spacial light modulation device suitable for use as a light information processing device incorporated in a projection display unit or a light computer.
2. Description of the Prior Art
Conventionally, there have been known projection display units of various types. In particular, a reflection-type spacial light modulation device is disclosed in Japanese Published Unexamined (Kokai) Patent Application No. 3-192332, by which modulated and reflected light can be obtained by changing the status of a liquid crystal sealed on a dielectric reflection layer side, on the basis of a voltage applied to respective pixel electrodes interposed between a photo-conductive layer and the dielectric reflection layer.
In more detail, as shown in FIG. 1, the spacial light modulation device is of laminated structure composed of a transparent drive electrode layer 1, a photoconductive layer 2, a pixel electrode layer 5 (in which a great number of pixel electrodes 4 are partitioned by an insulating substance layer 3 and arranged at a predetermined pitch), a dielectric reflection layer 6, a liquid crystal layer 7 (of a predetermined thickness determined by a spacer 12a, 12b), and a transparent drive electrode layer 8. In practice, the above-mentioned laminated structure is sandwiched between two transparent insulating (glass) substrates 9 and 10.
In the above-mentioned structure, under the condition that a rectangular wave voltage is applied between the two drive electrode layers 1 and 8 of the spacial light modulation device by a drive voltage supply 11 connected between the two transparent drive electrode layers 1 and 8, when write light FA is allowed to be incident upon the drive electrode layer 1, light information can be accumulated as charges in the respective pixel electrodes 4. On the other hand, when read light FB is allowed to be incident upon the drive electrode layer 8; reflected from the dielectric reflection layer 6; and further passed through the liquid crystal layer 7 as reflected light FC, since the read light FB incoming into the liquid crystal layer 7 is modulated by a voltage applied to the liquid crystal layer 7 through the dielectric reflection layer 6 on the basis of the charges accumulated in the respective pixel electrodes 4, it is possible to obtain modulated and reflected light outgoing from the drive electrode layer 8.
In the above-mentioned structure, the pixel electrodes 4 are arranged into a matrix pattern. In more detail, as shown in FIG. 2, a plurality of square-shaped pixel electrodes 4 each having a one-side length L1 are arranged at regular intervals of D1, that is, at a constant pitch P=(L1+D1) in two dimensional plane.
Further, since the modulation rate increases with increasing voltage applied to the respective pixel electrodes 4, it is possible to increase the image contrast of the reflected light FC. On the other hand, since the density of the reflected light FC at the respective pixels increases with decreasing pitch P (=L1+D1) between the respective pixel electrodes 4, it is possible to increase the resolution of the obtained image of the reflected light FC to that extent.
In the above-mentioned light modulation device, however, the voltage applicable to the respective pixel electrodes 4 is limited by energy barrier formed between the adjacent pixel electrodes 4, so that it is impossible to increase the applied voltage unconditionally.
In more detail, when the potential difference between the respective pixel electrodes 4 becomes higher than the energy barrier, the charges accumulated at the respective pixel electrodes 4 flow to the adjacent pixel electrodes 4 through the photoconductive layer 2 (on the side of Schottky contact surface), with the result that the resolution in unit of pixel of the modulated and reflected light FC deteriorates. Accordingly, it is necessary to reduce the voltage applied to the respective pixel electrodes 4 below the above-mentioned energy barrier.
On the other hand, when the carrier concentration of the photoconductive layer 2 is assumed to be constant, since the strength of the above-mentioned energy barrier is dependent upon the intervals D1 between the two adjacent pixel electrodes 4, it is possible to increase the energy barrier by increasing the intervals D1. Therefore, when the intervals D1 is increased, a high voltage can be applied to the pixel electrodes 4, so that the image contrast can be increased without deteriorating the resolution in unit of pixel.
In this case, however, when only the pitch P is increased while keeping the one-side length L1 of the pixel electrode 4 at a constant value, the number of the pixels must be reduced as a whole. In addition, since the aperture ratio (the opening ratio in area of the pixel electrodes 4 to the pixel electrode layer 5) is reduced at the modulation portion, the pixel density and the resultant resolution of the device itself both inevitably deteriorate.
In contrast with this, when the one-side length L1 of the respective pixel electrodes 4 is reduced to increase the intervals D1 (between the two opposing side surfaces of the pixel electrodes 4) without changing the arrangement pitch P of the pixel electrodes 4, since the junction area between the respective pixel electrodes 4 and the dielectric reflection layer 6 decreases, the aperture ratio at the modulation element area is inevitably reduced, so that the image contrast and the quantity of the modulated and reflected light FC are both lowered.
In other words, with respect to the improvement of the spacial light modulation device, there exists a contradictory relationship among the contrast, the aperture ratio and the resolution, so that it has been difficult to allow these conditions to be compatible, in particular in the case of the spacial light modulation device in which the pixel electrodes 4 are arranged at a pitch P on the order of 10 .mu.m.
In addition, the prior art spacial light modulation device as shown in FIG. 1 involves the other following problems: To form the pixel electrode layer 5, it is necessary to form an insulating layer on the photoconductive layer 2 and then to apply a photoresist thereon. Further, the insulating layer is etched with the use of the photoresist as a mask to form a plurality of apertures.
Further, after etching, a metal film is formed on all the surface of the photoresist remaining on the insulating layer and the surfaces of the apertures, and then only the metal formed on the side surfaces of the remaining photoresist is etched in accordance with lift-off method to remove the photoresist and the metal formed on the surface of the photoresist. As a result, the metal can be buried in the apertures formed in the insulating layer as the pixel electrodes 4.
In the above-mentioned process, the lift-off method is adopted to form the pixel electrode layer 5. Therefore, in the etching process thereof, there exists a tendency that the sides of the apertures are easily etched, irrespective of dry etching or wet etching. As a result, as shown in FIG. 3, a V-shaped groove 15 is formed between the pixel electrode 4 and the insulating substance layer 3 respectively by the side etching, so that the dielectric reflection layer 6 (a multilayer formed on the surface of the pixel electrode layer 5) is also formed into a V-shape groove 16 along the groove 15.
In other words, since a number of grooves 16 are formed on the surface of the dielectric reflection layer 6 and thereby the surface of the layer 6 is not flat, whenever the read light FB is allowed to be incident upon the dielectric reflection layer 6, the incident angles thereupon differ and therefore become abnormal at the respective grooves 16.
On the other hand, the dielectric reflection layer 6 is provided with such a function that the reflection factor is enhanced on the basis of interference of the light FC reflected from the respective interfaces of the multilayer films of the dielectric reflection layer 6. Further, the reflection factor of the dielectric reflection layer 6 is determined on the basis of the relationship between the wavelength of the read light FB and the film thickness in the direction that the read light FB transmits.
As a result, when a number of the grooves 16 are formed on the surface of the dielectric reflection layer 6 as described above, the reflection factor of the dielectric reflection layer 6 is reduced below the value designed on the assumption that the dielectric reflection layer 6 is flat. In addition, since the light scattering phenomenon is produced at the respective grooves 16, the brightness of the image due to the reflection light FC is lowered markedly.
Further, since a defectiveness (e.g., pinholes) is easily produced at the respective grooves 16 during the process of forming the dielectric reflection layer 6, the read light FB is reflected toward the horizontal direction of the surface of the dielectric reflection layer 6 or the vertical direction of the surface of the pixel electrodes 4 as shown by thin arrows in FIG. 3, with the result that the resolution of the image obtained on the basis of the reflected light FC deteriorates. In particular, since the reduction of the reflection factor at the respective grooves 16 conversely causes an increase of the transmission of the read light FB through the dielectric reflection layer 6, when the V-shaped grooves 15 formed at the peripheries of the pixel electrodes 4 are deep enough to reach the photoconductive layer 2, the read light FB leaks toward the photoconductive layer 2. In this case, since the charges are accumulated in the pixel electrodes 4 by the photoelectric transfer of both the write light FA and the leaked read light FB, when the liquid crystal layer 7 is light modulated by the charge thus accumulated, the image contrast is lowered markedly and further the light modulation is disabled at the worst.
On the other hand, if the anisotropic etching method is adopted in the process of forming the pixel electrode layer 5 in accordance with the lift-off method, it may be possible to prevent the side etching. In this method, however, since a reactive ion etching is required, there exists another problem in that the surface of the photoconductive semiconductor layer 2 is damaged due to ion impulses. Further, when defectiveness and impurities exist on the Schottky contact surface, since the diode characteristics deteriorate, it is rather difficult to adopt the anisotropic etching method.